The commercial importance of IMS for detecting analytes is reflected in the large number of patents related to detection of narcotics (U.S. Pat. No. 5,491,337), explosives (U.S. Pat. No. 6,225,623), contamination-indicating substances, chemical agent detection, and generally for monitoring release of hazardous gases to provide early warning of impending danger (U.S. Pat. No. 5,095,206). Contamination is a concern in many industries, ranging from the semiconductor, hard-disk drive, flat panel display, aerospace, and other high-tech industries. The damage inflicted by contamination is ubiquitous, causing problems to production processes, product material, equipment surfaces, and in serious cases can even affect human health. Accordingly, it is vital that contamination (or analytes indicative of narcotics, explosives or other hazards) be identified rapidly and reliably so that appropriate corrective steps are taken before significant damage occurs.
IMS systems are recognized for their utility in detecting analytes and can be readily deployed for continuous long-term monitoring of the surrounding environment. The general configuration of IMS systems is well known in the art, with such systems having a means for ionizing an analyte of interest and means for measuring ion mobilities by application of an electric field. Because different analytes may have different ion mobilities, IMS systems monitor and detect an analyte of interest by determining the speed with which ionized analyte moves through an applied electric field and interacts with an ion detector. There is ongoing effort to provide IMS systems having improved sensitivity and which are less prone to false positive readings, and particularly to overcome problems associated with presence of substances that may interfere with analyte monitoring and detection. As various means for increasing the sensitivity of IMS systems are developed, there is recognition that instrument selectivity can be accordingly impacted, such as by generation of anomalous peaks, charged particles and clusters thereof that can mask the signal used to detect an analyte of interest. A concern is that the analyte will not be detected, the calculated analyte concentration will be incorrect, or a false positive will trigger unnecessary action. Regardless, concern related to unreliable detection requires repeated testing and further delays in analyte detection. One technique for minimizing generation of anomalous signal from interfering substances involves introduction of dopants to the IMS.
To improve sensitivity while maintaining adequate selectivity, U.S. Pat. No. 5,491,337 proposes adding low concentration (on the order of a few ppm) of a dopant (nicotinamide vapor) to the carrier gas stream prior to introduction to the IMS detection cell (e.g., dopant is not added directly to the drift region). The dopant acts as a charge transfer mediator and assists in cleaning up the spectrum obtained in an IMS that detects narcotics obtained from air samples, thereby increasing system sensitivity. In that system, dopant is selected to exhibit proton affinity that is higher than most of the ions produced in the ionization chamber, so that a single peak is generated in the absence of narcotic vapors. The dopant molecules are preferably selected to have a basicity that is between the basicity of the hydrogen carrier and the alkaloid molecules of interest. In this manner, in the absence of non-alkaloid background at equilibrium, the ion spectrum shows only ion peaks associated with the dopant species. In the presence of narcotic vapors, charge transfer between the dopant molecules and narcotic molecules generates a population of narcotic ions which are detected. That system, however, is limited to ammonia (NH3) or nicotinamide dopants added in low concentration to the carrier and sample gas stream prior to introduction to the IMS cell.
U.S. Pat. No. 6,225,623 discloses an IMS that is doped with ions produced by a corona discharge ionization source for detecting explosive compounds and narcotics. There is recognition of an interfering peak problem when an analyte is introduced to the system. In a “clean” sample (without analyte and impurities), a single reactant ion peak is observed. In contrast, when the analyte is provided multiple, overlapping peaks are detected in addition to reactant ion peak. These other peaks tend to mask the reactant ion peak and decrease IMS sensitivity, as well as present possible false-positive problems. To overcome this interfering peak problem, chemical doping is used to change the way in which sample vapor introduced to the IMS is ionized and subsequently detected. See Proctor and Todd, Alternative Reagent Ions for Plasma Chromatography. Anal. Chem. 56:1794-97 (1984). In such dopant systems, the dopant is obtained by ionization by-products of the corona discharge ionization process (e.g., corona dopant ions) or by a chemical dopant source that recirculates. Such use of dopant reportedly suppresses background contamination without significant loss of ion peaks associated with the sample of interest (analyte in that case is RDX). That IMS, however, involves complicated pneumatics and closed-loop paths for introducing chemical dopant. In addition, detection of an analyte is by introducing a sample wipe that has swabbed a surface of interest and so requires high-temperature operation to satisfactorily detect an analyte of interest. High temperatures of about 250° C. are also required to reduce or eliminate water vapor in the system that would otherwise generate interfering peaks. Such high-temperature systems present design restrictions and limits the choice of surface materials to materials capable of withstanding such high temperatures.
A different IMS system known as high field asymmetric waveform ion mobility spectrometry (FAIMS) is disclosed in U.S. Pat. No. 7,026,612. In such systems, the applied electric field is switched between high and low voltage states to generate an asymmetric voltage waveform. Many FAIMS devices use a carrier gas comprising a purified flow of nitrogen, oxygen or dehumidified air (e.g., see U.S. Pat. No. 5,420,424). The carrier gas can be dehumidified by a filter or membrane that prevents passage of water vapor to the IMS cell. U.S. Pat. No. 7,026,612 discloses use of these filters in combination with dopant mixing of the sample prior to introducing the sample to the IMS cell, and more particularly prior to ionization of the mixture. In that system, the mixture contains less than about one percent dopant gas by volume and the carrier gas itself is a doped carrier gas. The dopant is not introduced directly to the separation region of the IMS cell to be transported by drift gas to the ionization region. In embodiments of that system without a water-removing filter, the carrier gas and sample has no traces of water or other contaminates that could adversely affect sensitivity and/or separation capability. This is a recognition that such systems remain prone to water vapor-induced generation of interfering ions and ion clusters.
Other IMS systems that use dopant to improve specificity are provided in U.S. Pat. Nos. 5,095,206, 5,032,721, 5,234,838, 5,095,206 and 5,283,199. U.S. Pat. No. 5,283,199 discloses using one or more dopants (e.g., methylamine) to improve detection of chlorine dioxide. Those systems generally require a membrane to exclude interfering substances or other means for minimizing water vapor and introduce the dopant to the carrier gas containing a gaseous analyte prior to introduction to the IMS cell. Such membranes add cost to the system and require maintenance to ensure they remain capable of removing adequate amount of unwanted substances while continuing to permit passage of analyte of interest.
U.S. Pat. No. 6,495,824 discloses an IMS system having a plurality of reactant-containing reservoirs which can be reacted with a sample to form adducts with varying ion mobilities. In a similar fashion to the other IMS systems known in the art, that system also introduces reactant to the sample or is itself the carrier stream. Reactant is not added directly to the separation region.
From the forgoing, it is apparent there is a need in the art for IMS systems that avoid generation of unwanted ions and clusters thereof that affect the ability to reliably and sensitively detect analytes of interest. In such a system, the need for a water-vapor removing membrane is avoided, thereby decreasing the complexity of the system while maintaining sensitivity.